This invention relates generally to exhaust gas sensors, and specifically to exhaust oxygen sensors.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, oxygen sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, for example, to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically consists of an ionically conductive solid electrolyte material, a porous platinum electrode with a porous protective overcoat on the sensor's exterior exposed to the exhaust gases, and a porous electrode on the sensor's interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria-stabilized, zirconia-based electrochemical galvanic cell operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine's exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia electrolyte, according to the Nernst equation: ##EQU1##
where:
E=electromotive force PA1 R=universal gas constant PA1 F=Faraday constant PA1 T=absolute temperature of the gas PA1 p.sub.O.sub..sub.2 .sup.ref =oxygen partial pressure of the reference gas PA1 P.sub.O.sub..sub.2 =oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.
Prior art exhaust sensors have utilized solid electrolytes that are disposed as layers independent from supporting materials. Such a configuration requires more raw material and fabrication. What is needed in the art is an apparatus and method for incorporating electrolytes directly into substrate layers in a process that is preferably amenable to automation.